1. Technical Field
This disclosure relates to a VCC (Voice Call Continuity), and more particularly, to providing VCC subscribers receiving a roaming service with a method and device for handling CS calls (Circuit Switching calls).
2. Discussion of the Background Art
In general, a Voice Call Continuity (VCC) refers to a type of application, namely, a home IMS application which is capable of transporting voice calls between CS domain and IMS domain. As such, the VCC provides functions of ‘voice-call-originations’, ‘voice-call-terminations’ and a ‘domain transfer’ from the CS domain to the IMS domain or vice versa. Here, the domain transfer refers to transferring access legs for voice calls toward a user equipment (UE) (i.e., a terminal) from the CS domain to the IMS domain or vice versa during an active session. The access leg denotes a call control leg between a VCC UE and Domain Transfer Function (DTF).
The domain transfer for a certain voice call/session from the CS domain to the IMS domain or vice versa is initiated only when a DTF is positioned (located) on a signal path of the voice call/session setup. For this, positioning of the DTF on the way of the signal path of the voice call/session setup is referred to as ‘anchoring in IMS’ or ‘anchoring’.
FIG. 1 illustrates architecture of a network for providing a VCC service.
As illustrated in FIG. 1, a VCC UE 10 denotes all types of terminals which can support the VCC service. The VCC UE 10 can access CS and PS domains. That is, when accessing the CS domain, the VCC UE 10 uses a UE-CS (not shown) provided therein, while using UE-IMS (not shown) provided therein when accessing the PS domain.
A VCC application 31 is an application server for providing the VCC service, and is constituted with entities which perform a series of functions. That is, the series of functions may include functions required to setup voice calls toward the VCC UE 10, and functions required to switch an access leg of the VCC UE 10 between the CS domain and the IMS domain with maintaining (performing) an active session. Here, the series of functions can be a domain transfer function, a domain selection function, a CS adaptation function, and CAMEL service. Detailed capabilities and operations for the series of functions are described in 3GPP TS 23.206 V1.2.0.
CS domain entities include Visited Mobile Switching Center (VMSC), Gateway MSC (GMSC), gsmSCF, and the like. IMS domain entities include P-CSCF (Proxy Call Session Control Function), S-CSCF (Serving Call Session Control Function), I-CSCF (Interrogating Call Session Control Function) and Media Gateway Control Function (MGCF).
Origination and termination of voice calls in the VCC service are briefly described as follows.
When a user of a VCC UE 10 (i.e., VCC service subscriber) originates a voice call by dialing a called party number, the voice call is first routed to a home network server subscribed by the user, namely, the VCC application 31. Even when the user dials the number in a roaming service area, the voice call originated (dialed) to the called party number by the user is routed to the VCC application 31 of the home network via a local network of the roaming service area. The VCC application 31 dials the voice call to the called party number via a CS or IMS domain.
As such, since every voice call originated by the user should be routed to the VCC application 31 of the user's home network, it is necessary to discriminate whether the called party number is a local number on a local network (e.g., on a certain local network in an overseas roaming service area) in which the user is currently positioned, or a number on a home network.
Also, if the user is currently located in an overseas roaming service area (i.e., a coverage area of a visited network) and the called party number originated by the user is the local number of the overseas roaming service area (i.e., a coverage area of a visited network), the VCC application 31 should reroute the originated voice call to the local network (i.e., the visited network). That is, the originated voice call may be routed to the home network and then may be coming back to the visited network (i.e., the network corresponding to the overseas roaming service area where the user is visiting).
In case where the VCC application 31 receives a called party number which is not in an international format, if an originating user (i.e., VCC subscriber) who has subscribed to the home network is receiving a roaming service via a network in a certain area abroad, namely, via a visited network, the VCC application 31 can not properly connect the originated calls to the called party number. That is, under a state in which a call originated by an originating user (e.g., a VCC subscriber who is roaming abroad) is routed to the VCC application 31 (i.e., a VCC application server of a home network subscribed by the originating user), if the called party number dialed by the originating user is a local number within the roaming area (i.e., the visited network), a problem may occur which the dialed called party number may be interpreted as a local number within the home network (e.g., Home PLMN). That is, the VCC application 31 (i.e., a server subscribed by the originating user and routing the user originated call) may interpret the local number originated by the originating user in a roaming area (i.e., the visited network) as if the location number in the roaming area is a local number of a home network even though the originating user intends to have dialed the local number served by the visited network (i.e., an overseas local network which the user is in).